1. Field of the Invention
This invention relates generally to multiple compressed air supply systems for an engine in a motor vehicle or a heavy industrial machine, for example, and more particularly to a multiple compressed air supply system including (1) a supercharger driven by engine output power via a driving power transmitting mechanism interconnecting the engine with an electro-magnetically operated clutch of the supercharger, and (2) a turbocharger having a turbine driven by engine exhaust gas and a compressor driven by the turbine.
2. Description of the Prior Art
Generally, with a multiple compressed air supply system it is possible to supply compressed air in all speed ranges of an engine by effectively utilizing a high performance supercharger for supplying compressed air in a low speed range of the engine, and utilizing a high performance turbocharger for supplying compressed air in a high speed range of the engine.
A prior art multiple compressed air supply system is disclosed in Japanese Laid-open Patent No. 58(1983)-222919 published on Dec. 24, 1983. The basic structure of a multiple compressed air supply system in that prior art involves a supercharger and a turbocharger able to be connected to each other in series or parallel by a three-way valve and a dumping mechanism. However, the switching between series and parallel connections is not performed in accordance with the driving condition of the supercharger and the turbocharger, so the compressed air pressure may be excessively changed upon switching, and the switching action is not smooth.
Another prior art multiple compressed air supply system is disclosed in Japanese Laid-open Patent No. 62(1987)-101834 published on May 12, 1987. The basic structure of a multiple compressed air supply system in the latter prior art involves a supercharger and a turbocharger interconnected in series and a control valve is disposed in a bypass passage connected to the supercharger. Switching from the supercharger and turbocharger dual operating condition into a supercharger-stopped condition is initiated upon the detection of a certain operating condition of a wastegate valve. When the supercharger and turbocharger are both operated, the turbocharger operation is reduced by an opening of the wastegate valve; therefore, a timing of the switching is delayed and the time for switching to be completed is lengthened whereby too much driving power is lost. Furthermore, the turbocharger is disposed upstream of the supercharger, whereupon during a period of low compressed air pressure in the turbocharger (e.g. during a low speed range of the engine), full use of the supercharger is not realized because the turbocharger is operated as an inhalation resistor such that a vacuum is generated at the turbocharger compressor which sucks oil into an intake air passage portion from the compressor.